The present invention relates to a method of controlling the operation of an approach system of a paper machine or the like web formation apparatus. Especially preferably the method of controlling the operation of an approach system according to the invention is applicable to be used in connection with paper and paperboard machines as well as in connection with various apparatuses performing non-woven webformating.
Almost all prior art paper machine approach systems, which are well described in, e.g. U.S. Pat. No. 4,219,340, comprise the following components: a mixing tank, a feed/mixing pump, a centrifugal cleaning plant, a gas separation tank, a head box feed pump, a head box screen, a paper machine head box and white water trays. Said components are placed in connection with the paper machine and arranged to operate as follows: The fiber material used for paper making and fillers which are diluted with so-called white water obtained from the paper machine, mostly from the wire part thereof, are dosed into the mixing tank often referred to as wire pit and located at the bottom level of the mill. By means of the feed/mixing pump also located at the bottom level of the mill, the fiber suspension is pumped from the mixing tank to the centrifugal cleaning plant located usually at the machine level of the mill, i.e. the location level of the paper machine, or, as in said patent, above it. By means of pressure created by said mixing pump, the fiber suspension accepted by the centrifugal cleaning plant is further conveyed to the gas separation tank located at a level above the machine level. From the gas separation tank the fiber suspension, wherefrom gas has been removed as thoroughly as possible, flows to the head box feed pump located at the bottom revel of the mill, which pump pumps the fiber suspension to the head box screen (not shown in said US patent) also located at the bottom level of the mill, wherefrom the fiber suspension flows to the machine level into the head box of the paper machine.
In order to operate, the gas separation tank described both in said U.S. Pat. No. 4,219,340 and in form of a somewhat newer modification in U.S. Pat. No. 5,308,384 requires a vacuum system most often comprising a vacuum pump, most commonly a so-called liquid ring pump, located at the same level as the gas separation tank, and a drop separator for removing liquid drops possibly present in the gas withdrawn by the vacuum pump. The actual gas separation tank is traditionally a large essentially horizontal tank into which the fiber suspension coming from the centrifugal cleaning plant is sprayed via separate injection pipes. The purpose of the spraying is to give the gas in bubble form a possibility to leave the fiber suspension as early as this stage. In most cases, the gas separation tank is further provided with an intermediate wall, a so-called overflow weir meant for stabilizing the surface level of the fiber suspension in the tank, although there are gas separation tanks with no overflow. The objective of keeping the surface level constant is to ensure a constant inlet pressure for the head box feed pump, at the same time ensuring a constant flow of paper pulp to the head box. In other words, the amount of fiber suspension fed to the gas separation tank via the centrifugal cleaning plant is always somewhat greater than required by the head box. The excess fiber suspension is led via the overflow weir usually to the other end of the gas separation tank, wherefrom a return pipe leads to the mixing tank. Fiber suspension to be pumped to the head box is obtained via a discharge outlet arranged at the bottom of the head box and led to the head box feed pump. Various gas separation tank solutions are disclosed in e.g. U.S. Pat. Nos. 5,236,475, 4,478,615, 4,455,224, 3,538,680, 2,717,536, 2,685,937 and 2,642,950, the three last mentioned of which deal with a gas separation tank without overflow.
U.S. Pat. No. 2,717,536 discusses a gas separation apparatus wherein the fiber suspension flow coming from the centrifugal cleaning plant is led to a gas separation tank having no overflow weir but wherein the surface level is kept constant by means of a surface level transducer and a feed pump flow regulation valve controlled by said transducer. Further said publication presents the location of the gas separation tank at the machine level, i.e. the same level as the paper machine head box.
U.S. Pat. No. 2,685,937 also discloses a gas separation tank with no overflow. In the solution of said patent, there is a float arranged in the gas separation tank, which float follows the changes of the surface level of the fiber suspension. The movements of the float have a direct effect on the fiber suspension flow being fed into the tank, as they regulate the amount of fiber suspension being fed to the tank via the injection pipes.
Said prior art apparatuses have some disadvantages of which e.g. the following problems are worth mentioning.
Firstly, the surface level of the gas separation tank controlled by either an over-flow or various float solutions or other devices directly following the surface level does naturally remain constant, but that does not lead to what the actual purpose of the surface level regulation is, i.e. a constant inlet pressure of the head box feed pump. A reason for this Is that the density of the fiber suspension being pumped together with the surface level determine the inlet pressure. Said density, in turn, is effected by e.g. the filler content and gas content of the fiber suspension. Despite the fact that the filler content of the fiber suspension should be as constant as possible, there are some fluctuations in that. The fluctuations in the density are mostly caused by the gas content of the fiber suspension, which gas content may, in the worst case, vary to the extent of several per cents. Such great changes in the density of the fiber suspension lead to fluctuations in the pulp amount pumped by the head box feed pump, which again is reflected in fluctuations in the thickness of the final product.
In addition to that, prior art apparatuses are not capable of quickly responding to problems caused by e.g. a change in the speed of the machine. According to prior art, attempts were made to solve these problems in a way presented in the block diagram of FIG. 2, which describes a situation where the speed of the paper machine is either increased (the right side of the Figure) or decreased (the left side of the Figure), i.e. the production of the web formation apparatus is changed. In a prior art system, the head box slice flow is naturally changed first, as the production of the machine is controlled by means of it, whether calculated in terms of basis weight of the product or in tons produced by the machine. The starting point is to keep both the head box pressure and the product grammage constant despite changes in the speed of the machine. By means of prior art regulation system this is done so that as the speed of the paper machine increases the slice opening is widened in such a way that a constant pulp amount in proportion to the speed of the wire is continuously flowing from the slice opening (assuming that the head box pressure is constant). When the regulation system senses the widening of the head box slice opening in form of pressure decrease in the head box, the pressure will be increased by increasing the output of the head box feed pump. This in turn results in the lowering of the surface level in the gas separation tank, whereby the regulation system makes the mixing pump feed more pulp into the gas separation tank and the surface level of the tank returns to normal. A regulation arrangement of this type causes various pressure fluctuations in the approach system. Firstly, in order to keep the head box pressure constant as the surface level in the gas separation tank as well as the inlet pressure of the head box feed pump decrease, the capacity of the mixing pump is increased. When the regulation has reached the mixing pump, the mixing pump increases the feed of the gas separation tank, whereby the surface level thereof starts to rise. This causes the head box pressure to increase, which in turn leads to decreasing the capacity of the head box feed pump in order to stabilize the pressure. As the surface level in the gas separation tank has reached its set value, the regulation system guides the mixing pump to decrease the flow, which generates a new pressure effect in the head box. This time the head box pressure leaps downwards, because the head box feed pump has decreased its capacity to correspond to the rising surface of the gas separation tank. As the surface level no more rises, the inlet pressure of the head box feed pump does not rise either. The regulation system handles with this situation by increasing the capacity of the head box feed pump in order to increase the head box pressure to its set value. In practice said development brings forth the danger that the whole production produced during said regulation will turn to broke, as fluctuations in the head box pressure are directly reflected in basis weight fluctuations of the production. In a corresponding way, the consequent effects of decreasing the speed of the paper machine are seen in the left part of FIG. 2.
The basic reason for the problems is that various regulation operations are performed in delay, which means that obvious changes have already taken place either in the head box pressure, the surface level in the gas separation tank or both. In such a case compensating for these calls for changes in the opposite direction, the compensation of which in prior art is further performed in delay, which naturally leads to a situation where reaching the balanced stage takes an unreasonably long time.
Characterizing features of the method of controlling the approach system of a paper machine or the like according to the invention, which solve e.g. said problems, are disclosed in the appended claims.